Mausritter, Carin, and DURF would work well for this.

Mausritter is based on Into the Odd and features cute mice exploring a world of giant proportions.

Carin is also based on Into the Odd and is more straight forward fantasy.

DURF is really simple and fast. It's really quick to make a character and get to playing.

Maybe Fate Accelerated? You can jump right in, just pick a High Concept (What the character is mainly about), maybe a Trouble and approaches (how you approach problems) and leave the rest to fill in. You have Fate points, which you spend to improve rolls and which you get by getting into trouble / adventure. The players have more ability to affect the story than in D&D, and is usually easier to get into for beginners than for experienced rpg players - check Book of Hanz for tips.

Mausritter is a mix of some easy systems, very "kid friendly", it actually won some awards because of it. The Tiny D6 systems are also good and easy to learn: two dice, if you have an attribute that helps, use three.

check out https://www.adventure.game/

If you'd like to play in a Harry Potter like setting, check out Kids on Brooms.

Extra simple and perfect for a one shot or two: Definitely Wizards

Not quite as simple but I adore this game (free demo on the page): Wanderhome

Edit: just found out Wanderhome is included in this huge TTRPG bundle right now.

At 11 years old any system can work already. If she is interested, D&D or any other mainstream system will work just fine. You just have to guide them through character creation more. Probably limit some choices or even come with pre-created character sheets they can choose from.

But if you want something simple the Core Micro system would work. It's free and you can download it on itch https://as-if.itch.io/core-micro The rules are very open and can be used for any setting. The stats determine how many dice you roll and you just give them the target number. Depending on how close to the target they are, they will succeed or fail with either something positive or negative happening on top of it.

With a little homebrew, you could even turn it into a Harry Potter setting. Just add an additional 7 attributes that are named after the Harry Potter spell types (Charm, Jinx, Hex, etc.). And then she can either use traditional attributes for checks or magic. Which has the added benefit of giving her always 2 chances to succeed. For example, she has to jump across a gap. If she fails her Grace check and is falling she can still make a Transfiguration spell check to see if she can build a bridge out of something nearby.
And the skill list is already intended to be expanded upon, so you can add stuff like Quidditch, Potion Brewing, or Rune studies.